1. Field of the Invention
The present invention relates to a rotary airfoil blade for aircraft and more particularly the tip of such a blade.
2. Description of the Prior Art
It is known that, not only in hover flight but also in advancing flight, the tips of the blades of the rotor of an aircraft with rotating airfoil, particularly a helicopter, has an important influence on the aerodynamic functioning of the rotor. In fact, the blade tips form the zones in which the greatest dynamic pressures and marginal vortices appear, which generate noise and power losses.
Improvement of the blade tips may result in an appreciable improvement in the performances of the aircraft. This is why numerous studies have been carried out in this connection.
For example, in the French patent No. FR-A-2 473 983, a helicopter blade is described whose tip is tapered and swept rearwardly and, in addition, is downwardly slanted. With the total span ofthe blade equal to R, this tip extends over a span of 0.07 R and is bent about a chord of the blade disposed at 0.04 R from the endmost edge of the blade. The whole of the outermost portion of the blade forms, with respect to the rest of said blade, a downwardly directed dihedral angle whose value is chosen equal to 20.degree..
Thus, the blade described in said patent comprises a clear break at a distance from the axis of rotation equal to 0.96 R.
With such a downward break, of the blade tip, the author of this patent claims an improvement in the hover performances of the blade. In fact, he explains that the blade tip dihedral makes it possible to emit the end vortex lower than when the blade tip is aligned with the rest of the blade. Consequently, when the following blade of the airfoil passes through the position of the vortex generated by the preceding blade, this vortex is further away from the following blade, so that the interaction between this latter and the preceding blade vortex is reduced.; hover test results published subsequently by the author of this patent confirm power gains, for the same weight and in hover flight, of the order of 2 to 4%, due to the blade tip dihedral alone.
The purpose of the present invention is to improve a blade for a rotary airfoil with downwardly slanted tip not only in order to further improve the hover performances but also, and especially, to obtain better performances in advancing flight. It will be notd that in said patent, only the evolution of the chord of the profiles and the sweep of the tip are presented as having a beneficial effect on the power in advancing flight.